Colorado Reservoirs Research Articles

An evaluation of the relative size, body condition, and survival of triploid Walleye in the wild

AbstractObjectiveNatural dispersal or unauthorized introductions of nonnative predatory sport fish can undermine native species conservation and other fisheries management objectives. The use of reproductively sterile triploid fish may provide a lower risk stocking option (i.e., genetic and reproductive containment) for diversifying angling opportunities while possibly reducing angler motivation to illegally transplant fish. Management interest in triploid Walleye Sander vitreus is growing, and these fish are currently stocked by several fisheries agencies. However, little is known about the poststocking growth, body condition, and survival of triploid Walleye—information that is important for calibrating expectations and guiding policy regarding their use.MethodsWe compared the age‐specific size, body condition, and survival of subadult (i.e., ages 0–3) triploid Walleye to those of normal diploid conspecifics in two eastern Colorado reservoirs. Paired stockings of triploid and diploid Walleye occurred over 3 years. Both fry and fingerlings were stocked each year.ResultTriploids were generally smaller in length (6.6% smaller on average) and weight (20.6% smaller) compared to diploids across all age‐classes. Despite being smaller, triploids had similar body condition relative to diploids. On average, the survival of triploids from spring stocking to their first fall was 6.3× lower than that of diploids, but no differences in survival were observed beyond this early life stage. This pattern was consistent across cohorts and reservoirs.ConclusionThe present work provides a foundation for future studies that are designed to clarify patterns of survival from spring to the first fall by accounting for size at stocking and natural‐origin fish and studies that are planned to assess the poststocking performance of triploid Walleye independent of diploids.

Predicting sport fish mercury contamination in heavily managed reservoirs: Implications for human and ecological health.

Mercury (Hg) is a concerning contaminant due to its widespread distribution and tendency to accumulate to harmful concentrations in biota. We used a machine learning approach called random forest (RF) to test for different predictors of Hg concentrations in three species of Colorado reservoir sport fish. The RF approach indicated that the best predictors of 864 mm northern pike (Esox lucius) Hg concentrations were covariates related to salmonid stocking in each study system, while system-specific metrics related to productivity and forage base were the best predictors of Hg concentrations of 381 mm smallmouth bass (Micropterus dolomieu), and walleye (Sander vitreus). Protecting human and ecological health from Hg contamination requires an understanding of fish Hg concentrations and variability across the landscape and through time. The RF approach could be applied to identify potential areas/systems of concern, and predict whether sport fish Hg concentrations may change as a result of a variety of factors to help prioritize, focus, and streamline monitoring efforts to effectively and efficiently inform human and ecological health.

Altitude diving on a closed-circuit oxygen rebreather: A case report

Closed-circuit rebreather diving is becoming more common. Rebreathers are complicated, adding to the stress of diving. Also adding to this complexity in the presented case is diving at a high-altitude cold-water reservoir in Colorado. One diver experienced an oxygen-induced seizure at depth. The other diver had a rapid ascent with loss of consciousness. As will be seen in this case, two experienced divers were able to recover from a possible devastating dive. Fortunately, they both returned to their pre-dive baseline health. Dive planning is important, but as in this case, dive execution is paramount. This is a clinical case for an uncommon event that presented to an emergency department.

Rapid proliferation of the parasitic copepod, Salmincola californiensis (Dana), on kokanee salmon, Oncorhynchus nerka (Walbaum), in a large Colorado reservoir.

Ecologically and economically valuable Pacific salmon and trout (Oncorhynchus spp.) are widespread and susceptible to the ectoparasite Salmincolacaliforniensis (Dana). The range of this freshwater copepod has expanded, and in 2015, S.californiensis was observed in Blue Mesa Reservoir, Colorado, USA, an important kokanee salmon (O.nerka, Walbaum) egg source for sustaining fisheries. Few S.californiensis were detected on kokanee salmon in 2016 (<10% prevalence; 2 adult S.californiensis maximum). By 2020, age-3 kokanee salmon had 100% S.californiensis prevalence and mean intensity exceeding 50 adult copepods. Year and kokanee salmon age/maturity (older/mature) were consistently identified as significant predictors of S.californiensis prevalence/intensity. There was evidence that S.californiensis spread rapidly, but their population growth was maximized at the initiation (the first 2-3years) of the invasion. Gills and heads of kokanee salmon carried the highest S.californiensis loads. S.californiensis population growth appears to be slowing, but S.californiensis expansion occurred concomitant with myriad environmental/biological factors. These factors and inherent variance in S.californiensis count data may have obscured patterns that continued monitoring of parasite-host dynamics, when S.californiensis abundance is more stable, might reveal. The rapid proliferation of S.californiensis indicates that in 5years a system can go from a light infestation to supporting hosts carrying hundreds of parasites, and concern remains about the sustainability of this kokanee salmon population.

Phytoplankton Drivers of Dissolved Organic Material Production in Colorado Reservoirs and the Formation of Disinfection By-Products

Previous studies have shown that algal-derived dissolved organic matter (DOM) has a strong influence on the formation of disinfection byproducts (DBPs) during the treatment of drinking water. In the summer of 2010, we evaluated the role of nitrogen and phosphorus loading and phytoplankton abundance as drivers of the concentrations and quality of DOM and the associated DBP formation in 30 reservoirs in the mountains and plains of the State of Colorado. Optical properties such as Specific Ultraviolet Absorbance at 254 nm (SUVA254) and fluorescence spectroscopy were used to characterize DOM quality. Nutrient concentrations such as total nitrogen were also assessed and were associated with high concentrations of chlorophyll a (Chl-a). In turn, high total organic carbon (TOC) concentrations were associated with high concentrations of Chl-a, and the DOM in these reservoirs had a fluorescence signature indicative of contributions from phytoplankton growth. The reservoirs with TOC concentrations above 4 mgC/L were predominantly located in the plains and many are impacted by agricultural runoff and wastewater discharges, rather than in the mountains and are characterized by warm water conditions and shallow depths. For a subset of fourteen reservoirs, we characterized the composition of the phytoplankton using a rapid imaging microscopy technique and observed a dominance by filamentous Cyanobacteria in reservoirs with TOC concentrations above 4 mgC/L. The combination of high TOC concentrations with microbial characteristics resulted in high potential for production of two major classes of regulated DBPs, trihalomethanes and haloacetic acids. While fluorescence spectroscopy was useful in confirming the contribution of phytoplankton growth to high TOC concentrations, evaluation of predictive models for DBP yields found that all equally predictive models included SUVA254 and some of these models also included fluorescence indices or logTOC. These findings provide a limnological context in support of the recent guidelines that have been implemented for protection of high-quality drinking water supplies in the State of Colorado.

Size‐Dependent Retention of Pelagic‐Oriented Kokanee in Multimesh Gill Nets

AbstractRecent studies evaluating the size selectivity of standard, horizontal, benthic gill nets for some common sport fish have demonstrated congruence across regions and gear configurations. Studies evaluating retention in gill nets designed to sample pelagic‐oriented fish (e.g., vertical and curtain nets) are rare, which has limited corresponding assessments of generality. This study estimated the retention of kokanee Oncorhynchus nerka (lacustrine Sockeye Salmon) in multimesh vertical gill nets using catch data compiled over three decades from a large montane reservoir in Colorado. The corresponding best‐fit retention model was compared with that developed for kokanee in Idaho using multimesh curtain nets. There was strong congruence (Pearson's r = 0.98) between the total retention values (integrated over all mesh sizes) estimated by the two models for kokanee 50–430 mm total length, and the values followed a 1:1 relationship. This result indicated that the retention of fish in pelagic gill nets may also be largely conserved across regions and gear configurations. Thus, the retention models for kokanee compared in this study should be flexible and broadly applicable. These properties enable practitioners to easily adjust length‐frequency distributions for bias arising from size‐dependent retention when minimally biased estimates of size structure are needed for addressing management or research questions.

Changes in Sport Fish Mercury Concentrations from Food Web Shifts Suggest Partial Decoupling from Atmospheric Deposition in Two Colorado Reservoirs

Partial decoupling of mercury (Hg) loading and observed Hg concentrations ([Hg]) in biotic and abiotic samples has been documented in aquatic systems. We studied two Colorado reservoirs to test whether shifts in prey for sport fish would lead to changes in [Hg] independent of external atmospheric Hg deposition. We compared sport fish total mercury concentrations ([T-Hg]) and macroinvertebrate (chironomids and crayfish) methylmercury concentrations ([MeHg]) before and after food web shifts occurred in both reservoirs. We also monitored wet atmospheric Hg deposition and sediment [T-Hg] and [MeHg] at each reservoir. We found rapid shifts in Hg bioaccumulation in each reservoir's sport fish, and these changes could not be attributed to atmospheric Hg deposition. Our study shows that trends in atmospheric deposition, environmental samples (e.g., sediments), and samples of species at the low trophic levels (e.g., chironomids and crayfish) may not accurately reflect conditions that result in fish consumption advisories for high trophic level sport fish. We suggest that in the short-term, monitoring fish [Hg] is necessary to adequately protect human health because natural and anthropogenic perturbations to aquatic food-webs that affect [Hg] in sport fish will continue regardless of trends in atmospheric deposition.

Regulating Chlorophyll a to Control DBP Precursors in Water Supply Reservoirs

Colorado recently adopted a chlorophyll a standard of 5 μg/L as an option for additional protection of direct‐use water supply reservoirs. Setting a limit on algal abundance by regulating chlorophyll a in these reservoirs is a preventive measure aimed at controlling or reducing algal‐derived precursors of disinfection by‐products (DBPs). Development of the chlorophyll a standard was based on field studies of Colorado reservoirs and compliance data from the associated water treatment systems. Controlling algal abundance in these reservoirs can supplement conventional treatment or serve as an alternative to additional treatment in situations in which algae contribute significantly to the organic precursors that can form DBPs during treatment. The focus on algae is important because algal‐derived precursors are difficult to remove during conventional water treatment and because algal abundance can be controlled via the nutrient supply.

Effects of prey assemblage on mercury bioaccumulation in a piscivorous sport fish

Mercury (Hg) is a persistent global contaminant that biomagnifies, often reaching maximum levels in apex predators. Mercury contamination in piscivorous fish is a serious health risk for anglers and other fish consumers. We used data collected from a reservoir in Colorado to develop bioenergetics-based simulations of Hg bioaccumulation to estimate Hg concentrations in walleye (Sander vitreus), a popular sport fish. We evaluated how changes in the prey available to walleye might affect walleye Hg concentrations. Our simulations showed that such changes could result in almost a 10-fold range in walleye Hg concentration. Walleye consuming invertebrates had low growth, low growth efficiency, and high Hg concentrations. Conversely, when walleye diet contained only fish prey their growth and growth efficiency were higher and Hg concentrations were about 85% lower. These predictions were consistent with independent measurements in the study system observed under two different prey regimes in 2008 and 2013. Because prey assemblages in freshwaters can exhibit high natural and anthropogenic variability, understanding variation in predator Hg and providing accurate fish consumption advice to anglers and their families will require frequent monitoring of both predator and prey species. Further, manipulation of prey assemblages is a routine fishery management strategy that could be applied to reduce Hg contamination in piscivorous fishes.

Mercury deposition and methylmercury formation in Narraguinnep Reservoir, southwestern Colorado, USA

Narraguinnep Reservoir in southwestern Colorado is one of several water bodies in Colorado with a mercury (Hg) advisory as Hg in fish tissue exceed the 0.3μg/g guideline to protect human health recommended by the State of Colorado. Concentrations of Hg and methyl-Hg were measured in reservoir bottom sediment and pore water extracted from this sediment. Rates of Hg methylation and methyl-Hg demethylation were also measured in reservoir bottom sediment. The objective of this study was to evaluate potential sources of Hg in the region and evaluate the potential of reservoir sediment to generate methyl-Hg, a human neurotoxin and the dominant form of Hg in fish. Concentrations of Hg (ranged from 1.1 to 5.8ng/L, n=15) and methyl-Hg (ranged from 0.05 to 0.14ng/L, n=15) in pore water generally were highest at the sediment/water interface, and overall, Hg correlated with methyl-Hg in pore water (R2=0.60, p=0007, n=15). Net Hg methylation flux in the top 3cm of reservoir bottom sediment varied from 0.08 to 0.56ng/m2/day (mean=0.28ng/m2/day, n=5), which corresponded to an overall methyl-Hg production for the entire reservoir of 0.53g/year. No significant point sources of Hg contamination are known to this reservoir or its supply waters, although several coal-fired power plants in the region emit Hg-bearing particulates. Narraguinnep Reservoir is located about 80km downwind from two of the largest power plants, which together emit about 950kg-Hg/year. Magnetic minerals separated from reservoir sediment contained spherical magnetite-bearing particles characteristic of coal-fired electric power plant fly ash. The presence of fly-ash magnetite in post-1970 sediment from Narraguinnep Reservoir indicates that the likely source of Hg to the catchment basin for this reservoir has been from airborne emissions from power plants, most of which began operation in the late-1960s and early 1970s in this region.

Tiger muskellunge predation on stocked salmonids intended for recreational fisheries

Hatchery-reared fish are stocked widely to enhance recreational fisheries but are often consumed by predators. Stable isotope analyses were used to evaluate tiger muskellunge (northern pike [Esox lucius] × muskellunge [E. masquinongy]) predation on stocked salmonids (Oncorhynchus) relative to naturally reproducing white suckers (Catostomus commersonii), in 5 Colorado reservoirs. Stable isotope analyses coupled with a mixing model using a Bayesian framework indicated that tiger muskellunge primarily consumed stocked salmonids (53–84% by mass). These results suggest that stocking salmonids into systems that contain tiger muskellunge (and potentially other predators) may result in losses of valuable stocked fish. Further, the use of tiger muskellunge or other piscivores as biological control of less desirable species to benefit sympatric salmonid populations may be counterproductive to management goals. Finally, this study demonstrates the potential for managers to use this framework as a tool to identify and evaluate unintended losses of fishes to piscivores in other systems.

Prevalence and intensity of the parasitic copepod (Salmincola californiensis) on Kokanee salmon (Oncorhynchus nerka) in a reservoir in Colorado

Copepods of the genus Salmincola (gill lice) parasitize salmonids. We collected kokanee salmon (Oncorhynchus nerka) from a reservoir in Colorado to identify the species of gill lice present and investigate intensity and prevalence of infestations. We observed increasing intensity and prevalence with age of fish. Our study adds to limited knowledge of infestations of Salmincola in Colorado and the western United States.

Fracture parameter estimation from well-log data

We evaluated a method of deriving seismic fracture parameters from vertical-well-log data with the assumption that the fractured medium is transversely isotropic with a horizontal axis of symmetry (HTI). One approximation we used is that the observed vertical P-wave velocity is the same as the background isotropic P-wave velocity of the HTI medium. Another assumption was that the fractures and cracks are noninteractive and penny shaped. Using these approximations, we generated the fracture compliance matrix for each layer. Fracture parameters were then derived by constructing the HTI stiffness matrix for those layers. We tested our method using vertical-well-log data from a tight sand reservoir in Colorado, USA. “Thomsen-style” parameters were derived, and gas-filled fractures were identified on this log. The identified gas-filled fractures were compared to the production log data. The fracture density was also obtained at the well location within the depth of interest. We also found some problems and limitations caused by approximating vertical P-wave velocity the same as the background isotropic P-wave velocity.

Rainbow Trout and Their Hybrids Out compete Cutthroat Trout in a Lentic Ecosystem?

Much has been written about introduced rainbow trout interbreeding and outcompeting native cutthroat trout. However, the specific mechanisms by which rainbow trout and their hybrids outcompete cutthroat trout have not been thoroughly explored, and most of the published data is limited to lotic ecosystems. Samples of Snake River cutthroat trout (Oncorhynchus clarkii bouvieri), the rainbow-cutthroat hybrid, the cutbow trout (Onchorhynchus mykiss x clarkii), and rainbow trout (Oncorhynchus mykiss), were obtained from a lentic ecosystem (Eleven Mile Reservoir, Colorado) by creel surveys conducted from May to October, 2012. The total length and weight of each fish was measured and the relative condition factor of each fish was computed using expected weight from weight-length relationships from the Colorado Division of Parks and Wildlife (CDPW). Data from the CDPW collected from 2003 ? 2010 in the same lentic ecosystem were used to compute relative condition factors for additional comparison, as was independent creel survey data from 2011. The data was also compared with minimum, 25th percentile, mean, 75th percentile, and maximum weight-length curves generated from independent North American data. Cutthroat trout were plump: the mean relative condition factor of the cutthroat trout was 112.0% (? 1.0%). Cutbow hybrid trout were close to the expected weights with a mean relative condition factor of 99.8% (? 0.6%). Rainbow trout were thinner with a mean relative condition factor of 96.4% (? 1.4%). Comparing mean relative condition factors of CDPW data from earlier years and plotting the 2012 data relative to percentile curves also shows the same trend of cutthroat trout being plumper than expected and rainbow trout being thinner than the cutthroat trout, with the hybrid cutbow trout in between. This data supports the hypothesis that rainbow trout do not outcompete cutthroat trout in lentic ecosystems. Comparison with data from three other Colorado reservoirs also shows that cutthroat trout tend to be more plump than rainbow trout and their hybrids in sympatric lentic ecosystems.

Otolith mass as a predictor of age in kokanee salmon (Oncorhynchus nerka) from four Colorado reservoirs

Estimating ages of individuals in fish populations is crucial for determining characteristics necessary to effectively manage sport fisheries. Currently, the most accepted approach for fish age determination is using thin sectioned otoliths for interpretation. This method is labor-intensive, requires extensive training, and subjectively determines age. Several studies have shown that otolith mass increases with age, yet use of otolith mass to determine fish age is relatively underutilized. However, determining fish age using otolith mass requires relatively little training, is relatively nonsubjective, and is faster compared with other aging techniques. We collected kokanee salmon (i.e., landlocked sockeye salmon, Oncorhynchus nerka ) in 2004 from four reservoirs and from 2000 to 2009 in one reservoir to evaluate the efficacy of using otolith mass to determine fish ages. We used a machine learning technique to predict kokanee salmon ages using otolith mass and various other covariates. Our findings suggest this method has potential to substantially reduce time and financial resources required to age fish. We conclude that using otolith mass to determine fish age may represent an efficient and accurate approach for some species.

Diversity of algal viruses in various North American freshwater environments

To examine algal virus (Phycodnaviridae) genetic diversity in freshwater environments, gene fragments were cloned and sequenced from a river and a reservoir in Colorado, USA, and 2 different lakes in Ontario, Canada using PCR methods that target a diverse subset of known Phycodnaviridae DNA polymerase genes. Numerous phycodnavirus gene sequences were obtained from every sample, and rarefaction analysis of the sequence libraries demonstrated that virus richness was variable among different sample locations, and among samples collected from the same location at different times. Phylogenetic analysis of the unique sequences from each sample indicated that most sequences from the same geographic region (i.e. Colorado or Ontario) clustered together, but several exceptions were also observed. Phylogenetic analysis also demonstrated that the sequences obtained were more closely related to sequences from cultivated marine phycodnaviruses belonging to the genus Prasinovirus than to those from cultivated freshwater phycodnaviruses from the genus Chlorovirus. Overall, phycodnavirus sequences originating from cultivated marine viruses and marine clone libraries were not genetically distinct from the freshwater phycodnavirus sequences reported in this study.

Application of a nutrient-saturation concept to the control of algal growth in lakes

Either phosphorus or nitrogen can be responsible for nutrient limitation of algae in lakes. Nitrogen limitation can be defeated by heterocystous cyanobacteria through nitrogen fixation, but no comparable mechanism exists for P. Therefore, P is considered the predominant factor limiting phytoplankton biomass in lakes. Even so, increasing numbers of studies show that many lakes are limited by N deficiency because heterocystous cyanobacteria do not become sufficiently abundant to offset N deficiency. Where N limitation prevails, P control over phytoplankton populations can be achieved only if P concentrations are first reduced to a saturation threshold that is determined by the amount of available N. The extent of this reduction, which will typically occur without any suppression of phytoplankton biomass, can be estimated from nutrient chemistry, nutrient enrichment experiments, and information on the stoi-chiometry of phytoplankton, as illustrated with data for a Colorado reservoir in which a reduction of N of about 50% would be necessary to induce P limitation. Analysis based on stoichiometry could allow managers of water quality in lakes to anticipate the implications of N limitation for P-based management of water quality.

UNDERSTANDING EASTERN BASKETMAKER II CHRONOLOGY AND MIGRATIONS

The Basketmaker II period in the upper San Juan River valley is historically defined from the study of two physiographic regions: one near Durango, Colorado, and the other near Navajo Reservoir in New Mexico and Colorado. The Los Pinos phase designation has been applied to the Basketmaker II period from both locations; however, recent archaeological investigations from these areas have identified temporal and spatial variations that challenge the usefulness of a single phase name to reflect such a long and dynamic Basketmaker II occupation. Chronometric, settlement, and climatic data are introduced as support for the proposal to separate the Basketmaker II occupation of the upper San Juan River valley into three phases—Animas, Archuleta, and Los Pinos. The Los Pinos phase terminology is retained but two regional subphases, the Talus Village subphase and the La Boca subphase, are introduced.

Influence of freshwater macrophytes on the littoral ecosystem structure and function of a young Colorado reservoir

The presence of rooted macrophytes, mostly the milfoil Myriophyllum sibiricum, was manipulated in enclosures in the littoral zone of a Colorado reservoir. The presence of macrophytes significantly increased the abundance of major invertebrate taxa by 70–1725% and increased the emission of methane 127%. The increase in abundance of most invertebrates was probably due to the habitat and surfaces provided by milfoil as stable isotope analyses indicated that milfoil was an insignificant carbon source for all of the invertebrate taxa, except for the milfoil midge Cricotopus myriophylli. Cricotopus is known to specialize on milfoil (other members of the genus specialize on Hydrilla or are generalists), had an isotopic signature that indicated a diet of milfoil, and was about 15 times more abundant when milfoil was present than when it was absent. Milfoil had no detectable effect on the total particulate phosphorus (TPP), soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP), dissolved organic phosphorus (DOP), and Chl a of water within the enclosures. However, enclosures containing milfoil had higher concentrations of SRP in the pore water of surface sediments than enclosures that had milfoil removed. SRP in pore water dropped below 2 μg/L at >2 cm sediment depth and DOP increased progressively from nearly zero at the surface to about 150 μg/L at 15 cm depth, regardless of vegetation. Thus, milfoil had significant effects on many, but not all, measures of littoral ecosystem structure and function that were monitored.

Do Predators Influence the Distribution of Age-0 Kokanee in a Colorado Reservoir?

Seasonal changes in reservoir conditions such as productivity, light, and temperature create spatiotemporal variation in habitat that may segregate or aggregate predators and prey, producing implications for the distribution, growth, and survival of fishes. We used hydroacoustics to document the diel vertical distribution of age-0 kokanee Oncorhynchus nerka relative to environmental gradients at Blue Mesa Reservoir, Colorado, during May–August of 2002. Temperature, light, and zooplankton density profiles were examined relative to foraging conditions for kokanee and their primary predator, lake trout Salvelinus namaycush. Age-0 kokanee displayed large diel vertical migrations in May despite the lack of an energetic advantage before reservoir stratification. Age-0 kokanee minimized near-surface foraging at this time, perhaps to avoid predation by visual predators, such as lake trout, in the well-lit surface waters. Strong reservoir stratification in midsummer appeared to provide a thermal refuge from lake trout that the kokanee exploited. By August vertical migrations were shallow and most kokanee remained in the epilimnion throughout the day. Although the energetic implications of the late-summer strategy are unclear, it appears that kokanee were responding to changes in their predator environment. A robust model for kokanee diel vertical migration across a range of systems should include a predator avoidance component.